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In biology and ecology, extinction is the end of
an organism or group of taxa. The moment of extinction is generally considered
to be the death of the last individual of that species (although
the capacity to breed and
recover may have been lost before this point). Because a
species' potential range may be very
large, determining this moment is difficult, and is usually done
retrospectively. This difficulty leads to phenomena such as
Lazarus taxa, where a species presumed
extinct abruptly "re-appears" (typically in the fossil record) after a period of apparent
absence.

Through evolution, new species arise
through the process of speciation—where
new varieties of organisms arise and thrive when they are able to
find and exploit an ecological
niche—and species become extinct when they are no longer able
to survive in changing conditions or against superior competition.
A typical species becomes extinct within 10 million years of its
first appearance, although some species, called living fossils, survive virtually unchanged
for hundreds of millions of years. Extinction, though, is usually a
natural phenomenon; it is estimated that 99.9% of all species that
have ever lived are now extinct.

Mass extinctions are relatively rare
events, however, isolated extinctions are not rare. Starting
approximately 100,000 years ago, and coinciding with an increase in
the numbers and range of humans, species extinctions have increased
to a rate estimated at 100—1000 times that in the recent fossil
record. This is known as the Holocene extinction and is at least the
sixth such extinction
event. Some experts have estimated that up to half of presently
existing species may become extinct by 2100.

Definition

A species becomes extinct when the last existing member of that
species dies. Extinction therefore becomes a certainty when there
are no surviving individuals that are able to reproduce and create
a new generation. A species may become functionally extinct when only a
handful of individuals survive, which are unable to reproduce due
to poor health, age, sparse distribution over a large range, a lack
of individuals of both sexes (in sexually reproducing species), or other
reasons.

In ecology, extinction is often
used informally to refer to local
extinction, in which a species ceases to exist in the chosen
area of study, but still exists elsewhere. This phenomenon is also
known as extirpation. Local extinctions may be followed by a
replacement of the species taken from other locations; wolf reintroduction is an example of
this. Species which are not extinct are termed extant. Those that are extant but threatened by
extinction are referred to as threatened
or endangered species.

An important aspect of extinction at the present time are human
attempts to preserve critically endangered species, which is
reflected by the creation of the conservation status"Extinct in the Wild" . Species listed
under this status by the World
Conservation Union (IUCN) are not known to have any living
specimens in the wild, and are maintained only in zoos or other artificial environments. Some of these
species are functionally extinct, as they are no longer part of
their natural habitat and it is unlikely the species will ever be
restored to the wild. When possible, modern zoological institutions attempt to maintain a
viable population for species
preservation and possible future reintroduction to the wild through use of
carefully planned breeding
programs.

The extinction of one species' wild population can have knock-on
effects, causing further extinctions. These are also called "chains
of extinction". This is especially common with extinction of
keystone species.

Pseudoextinction

Descendants may or may not exist for extinct species. Daughter
species that evolve from a parent species carry on most of the
parent species' genetic information, and
even though the parent species may become extinct, the daughter
species lives on. In other cases, species have produced no new
variants, or none that are able to survive the parent species'
extinction. Extinction of a parent species where daughter species
or subspecies are still alive is also called pseudoextinction.

Pseudoextinction is difficult to demonstrate unless one has a
strong chain of evidence linking a living species to members of a
pre-existing species. For example, it is sometimes claimed that the
extinct Hyracotherium, which
was an early horse that shares a common ancestor with the modern
horse, is pseudoextinct, rather than extinct,
because there are several extant
species of Equus, including
zebra and donkeys.
However, as fossil species typically leave no genetic material
behind, it is not possible to say whether Hyracotherium
actually evolved into more modern
horse species or simply evolved from a common ancestor with
modern horses. Pseudoextinction is much easier to demonstrate for
larger taxonomic groups.

Causes

There are a variety of causes that can contribute directly or
indirectly to the extinction of a species or group of species.
"Just as each species is unique," write Beverly and Stephen
Stearns, "so is each extinction... the causes for each are
varied—some subtle and complex, others obvious and simple". Most
simply, any species that is unable to survive or reproduce in its environment, and unable to
move to a new environment where it can do so, dies out and becomes
extinct. Extinction of a species may come suddenly when an
otherwise healthy species is wiped out completely, as when toxicpollution renders its
entire habitat unlivable; or may occur
gradually over thousands or millions of years, such as when a
species gradually loses out in competition for food to better
adapted competitors.

Assessing the relative importance of genetic factors compared to
environmental ones as the causes of extinction has been compared to
the nature-nurture debate. The
question of whether more extinctions in the fossil record have been caused by evolution or by catastrophe is a subject of
discussion; Mark Newman, the author of Modeling Extinction
argues for a mathematical model that falls between the two
positions. By contrast, conservation biology uses the extinction vortex model to classify
extinctions by cause. When concerns about human extinction have been raised, for
example in Sir Martin Rees' 2003 book
Our Final Hour, those
concerns lie with the effects of climate
change or technological
disaster.

Currently, environmental groups and some governments are concerned
with the extinction of species caused by humanity, and are
attempting to combat further extinctions through a variety of
conservation programs. Humans
can cause extinction of a species through overharvesting, pollution, habitat
destruction, introduction of new predators and food competitors, overhunting, and other influences.
According to the World
Conservation Union (WCU, also known as IUCN), 784 extinctions
have been recorded since the year 1500 (to the year 2004), the
arbitrary date selected to define "modern" extinctions, with many
more likely to have gone unnoticed (several species have also been
listed as extinct since the 2004 date).

Genetics and demographic phenomena

Population genetics and
demographic phenomena affect the evolution, and therefore the risk
of extinction, of species. Species with small populations are much more
vulnerable to these types of effects. Limited geographic range is
the most important determinant of genus
extinction at background rates but becomes increasingly irrelevant
as mass extinction arises.

Natural selection acts to
propagate beneficial genetic traits and eliminate weaknesses. It is
nevertheless possible for a deleterious mutation to be spread
throughout a population through the effect of genetic drift.

A diverse or deep gene pool gives a
population a higher chance of surviving an adverse change in
conditions. Effects that cause or reward a loss in genetic diversity can increase the chances
of extinction of a species. Population bottlenecks can
dramatically reduce genetic diversity by severely limiting the
number of reproducing individuals and make inbreeding more frequent. The founder effect can cause rapid,
individual-based speciation and is the most dramatic example of a
population bottleneck.

Genetic pollution

Purebred, naturally evolved, region specific wild species can be
threatened with extinction in a big way through the process of
genetic pollution—i.e., uncontrolled hybridization, introgression genetic swamping which leads to
homogenization or replacement of local genotypes as a result of a
numerical and/or fitness advantage
of the introduced plant or animal. Nonnative species can bring
about a form of extinction of native plants and animals by
hybridization and introgression, either through purposeful
introduction by humans or through habitat modification, bringing
previously isolated species into contact. These phenomena can be
especially detrimental for rare species coming into contact with
more abundant ones, where the abundant ones can interbreed with
them, swamping the entire rarer gene pool and creating hybrids,
thus driving the entire original purebred native stock to complete
extinction. Such extinctions are not always apparent from morphological (outward appearance)
observations alone. Some degree of gene
flow may be a normal, evolutionarily constructive process, and
all constellations of genes and genotypes cannot be preserved
however, hybridization with or without introgression may,
nevertheless, threaten a rare species' existence.

Widespread genetic pollution also leads to weakening of the
naturally evolved (wild) region specific gene pool leading to
weaker hybrid animals and plants which are not able to cope with
natural environs over the long run and fast tracks them towards
final extinction.

The gene pool of a species or a population is
the complete set of unique alleles that
would be found by inspecting the genetic material of every living
member of that species or population. A large gene pool indicates
extensive genetic diversity, which
is associated with robust populations that can survive bouts of
intense selection. Meanwhile, low genetic
diversity (see inbreeding and population bottlenecks) can cause
reduced biological fitness and an
increased chance of extinction amongst the reducing population of
purebred individuals from a species.

Habitat degradation

The degradation of a species' habitat may alter the fitness landscape to such an extent that
the species is no longer able to survive and becomes extinct. This
may occur by direct effects, such as the environment becoming
toxic, or indirectly, by limiting a
species' ability to compete effectively for diminished resources or
against new competitor species.

Habitat degradation through toxicity can kill off a species very
rapidly, by killing all living members through contamination or sterilizing them. It can also
occur over longer periods at lower toxicity levels by affecting
life span, reproductive capacity, or competitiveness.

Habitat degradation can also take the form of a physical
destruction of niche habitats. The widespread destruction of
tropical rainforests and
replacement with open pastureland is widely cited as an example of
this; elimination of the dense forest eliminated the infrastructure
needed by many species to survive. For example, a fern that depends on dense shade for protection from
direct sunlight can no longer survive without forest to shelter it.
Another example is the destruction of ocean floors by bottom trawling.

Diminished resources or introduction of new competitor species also
often accompany habitat degradation. Global warming has allowed some species to
expand their range, bringing unwelcome competition to other species
that previously occupied that area. Sometimes these new competitors
are predators and directly affect prey species, while at other
times they may merely outcompete vulnerable species for limited
resources. Vital resources including water and
food can also be limited during habitat degradation, leading to
extinction.

Predation, competition, and disease

Humans have been transporting animals and
plants from one part of the world to another
for thousands of years, sometimes deliberately (e.g., livestock released by sailors onto islands as a
source of food) and sometimes accidentally (e.g., rats escaping from boats). In most cases, such
introductions are unsuccessful, but when they do become established
as an invasive alien species, the
consequences can be catastrophic. Invasive alien species can affect
native species directly by eating
them, competing with them, and introducing pathogens or parasites that
sicken or kill them or, indirectly, by destroying or degrading
their habitat. Human populations may themselves act as invasive
predators. According to the "overkill hypothesis", the
swift extinction of the megafauna in areas
such as New
Zealand, Australia, Madagascar and Hawaii resulted
from the sudden introduction of human beings to environments full
of animals that had never seen them before, and were therefore
completely unadapted to their predation techniques.

Coextinction

Coextinction refers to the loss of a species due to the extinction
of another; for example, the extinction of parasitic insects following the loss of their
hosts. Coextinction can also occur when a species loses its
pollinator, or to predators in a food
chain who lose their prey. "Species coextinction is a
manifestation of the interconnectedness of organisms in complex
ecosystems ... While coextinction may not be the most important
cause of species extinctions, it is certainly an insidious one".
Coextinction is especially common when a keystone species goes extinct.

Global warming

There is also discussion about the long term affects of global
warming on the extinction process. Currently, studies have
concluded that global warming may drive one quarter of all land
animals and plants to extinction by 2050. The absolute worst case
scenario that we are facing is a thrilling 1/3 to 1/2 of all plant
and animal species facing extinction. Bhattacharya, Shaoni. "Global
warming threatens millions of species." Global

The ecologically rich hot spots where potentially most damage would
be done include places like South Africa's Cape Floristic Region,
and the Caribbean Basin. These areas include a doubling of present
carbon dioxide levels and rising temperatures that could eliminate
56,000 plant and 3,700 animal species in these hot spot
regions.

The white form of the lemuroid
possum, only found in the mountain forests of northern
Queensland, was once named as the first mammal species sub-form to
be driven extinct by global warming.
However since then 3 possums have been found. Also a more common
brown form of the lemuroid possum is
only considered "near threatened" and is not at risk of
extinction.Climate change however is only one threat to animal and
plant species. Plants and animals are also feeling the devastating
effects of deforestation and habitat destruction. Handwerk, Brian.
"Global Warming Could Cause Mass Extinctions by 2050, Study

Mass extinctions

There have been at least five mass extinctions in the history of
life on earth, and four in the last 3.5 billion years in which many
species have disappeared in a relatively short period of geological
time. The most recent of these, the Cretaceous–Tertiary extinction
event 65 million years ago at the end of the Cretaceous period, is best known for having wiped
out the non-avian dinosaurs, among many
other species.

Modern extinctions

According
to a 1998 survey of 400 biologists conducted by New York's American Museum
of Natural History, nearly 70 percent believed that they were
currently in the early stages of a human-caused extinction, known
as the Holocene
extinction. In that survey, the same proportion of
respondents agreed with the prediction that up to 20 percent of all
living populations could become extinct within 30 years (by 2028).
Biologist E.O.Wilson estimated
in 2002 that if current rates of human destruction of the biosphere
continue, one-half of all species of life on earth will be extinct
in 100 years. More significantly the rate of species extinctions at
present is estimated at 100 to 1000 times "background" or average
extinction rates in the evolutionary time
scale of planet Earth.

History of scientific understanding

In the 1800s when extinction was first described, the idea of
extinction was threatening to those who held a belief in the
Great Chain of Being, a
theological position that did not allow for
"missing links".

The possibility of extinction was not widely accepted before the
1800s. The devoted naturalist Carl
Linnaeus, could "hardly entertain" the idea that humans could
cause the extinction of a species. When parts of the world had not
been thoroughly examined and charted, scientists could not rule out
that animals found only in the fossil record were not simply
"hiding" in unexplored regions of the Earth. Georges Cuvier is credited with establishing
extinction as a fact in a 1796 lecture to the French
Institute.
Cuvier's observations of fossil bones convinced him that they did
not originate in extant animals. This discovery was critical for
the spread of uniformitarianism, and lead to
the first book publicizing the idea of evolution though Cuvier
himself strongly opposed the theories of evolution advanced by
Lamarck and others.

Human attitudes and interests

Extinction is an important research topic in the field of zoology, and biology in
general, and has also become an area of concern outside the
scientific community. A number of organizations, such as the
Worldwide Fund for
Nature, have been created with the goal of preserving species
from extinction. Governments have
attempted, through enacting laws, to avoid habitat destruction,
agricultural over-harvesting, and pollution. While many human-caused extinctions
have been accidental, humans have also engaged in the deliberate
destruction of some species, such as dangerous viruses, and the total destruction of other
problematic species has been suggested. Other species were
deliberately driven to extinction, or nearly so, due to poaching or
because they were "undesirable", or to push for other human
agendas. One example was the near extinction of the American
bison, which was nearly wiped out by mass
hunts sanctioned by the United States government, in order to force
the removal of Native
Americans, many of whom relied on the bison for food.

Biologist
Bruce Walsh of the University of Arizona states three reasons for scientific interest in the
preservation of species; genetic resources, ecosystem stability,
and ethics; and today the scientific
community "stress[es] the importance" of maintaining
biodiversity.

In modern times, commercial and industrial interests often have to
contend with the effects of production on plant and animal life.
However, some technologies with minimal, or no, proven harmful
effects on Homo sapiens can be
devastating to wildlife (for example, DDT).
BiogeographerJared Diamond notes that while big business may label environmental concerns
as "exaggerated", and often cause "devastating damage", some
corporations find it in their interest to adopt good conservation
practices, and even engage in preservation efforts that surpass
those taken by national parks.

Governments sometimes see the loss of native species as a loss to
ecotourism, and can enact laws with
severe punishment against the trade in native species in an effort
to prevent extinction in the wild. Nature preserves are created by governments
as a means to provide continuing habitats to species crowded by
human expansion. The 1992 Convention on Biological
Diversity has resulted in international Biodiversity Action Plan
programmes, which attempt to provide comprehensive guidelines for
government biodiversity conservation. Advocacy groups, such as The
Wildlands Project and the Alliance for Zero Extinctions, work to
educate the public and pressure governments into action.

People who live close to nature can be dependent on the survival of
all the species in their environment, leaving them highly exposed
to extinction risks. However, people prioritize
day-to-day survival over species conservation; with human overpopulation in tropical developing countries, there has been
enormous pressure on forests due to subsistence agriculture, including
slash-and-burn agricultural
techniques that can reduce endangered species's habitats.

Planned extinction

Humans have aggressively worked toward the extinction of many
species of viruses and bacteria in the cause of disease
eradication. For example, the smallpox
virus is now essentially extinct in the wild—although samples are
retained in laboratory settings, and the polio
virus is now confined to small parts of the world as a result of
human efforts to prevent the disease it causes.

Olivia Judson is one of six modern
scientists to have advocated the deliberate extinction of specific
species. Her September 25, 2003 New
York Times article, "A Bug's Death", advocates "specicide"
of thirty mosquito species through the
introduction of a genetic element, capable of inserting itself into
another crucial gene, to create recessive
"knockout genes". Her arguments for
doing so are that the Anopheles
mosquitoes (which spread malaria) and
Aedes mosquitoes (which spread
dengue fever, yellow fever, elephantiasis, and other diseases) represent
only 30 species; eradicating these would save at least one million
human lives per annum at a cost of reducing the genetic diversity of the familyCulicidae
by only 1%. She further argues that since species become extinct
"all the time" the disappearance of a few more will not destroy the
ecosystem: "We're not left with a
wasteland every time a species vanishes. Removing one species
sometimes causes shifts in the populations of other species —
but different need not mean worse." In addition, anti-malarial and mosquito control programs offer
little realistic hope to the 300 million people in developing nations who will be infected
with acute illnesses this year. Although trials are ongoing, she
writes that if they fail: "We should consider the ultimate
swatting."

Cloning

Recent technological advances have encouraged the hypothesis that
by using DNA from the remains of an extinct species, through the
process of cloning, the
species may be "brought back to life". Proposed targets for cloning
include the mammoth, thylacine, and the Pyrenean Ibex. In order for such a program to
succeed, a sufficient number of individuals would have to be
cloned, from the DNA of different individuals (in the case of
sexually reproducing organisms) to create a viable population.
Though bioethical and philosophical objections have been raised, the
cloning of extinct creatures seems a viable outcome of the
continuing advancements in our science and technology.

In 2003, scientists attempted to clone the extinct Pyrenean Ibex (C. p. pyrenaica). This initial
attempt failed; of the 285 embryos reconstructed, 54 were
transferred to 12 mountain goats and mountain goat-domesticated
goat hybrids, but only two survived the initial two months of
gestation before they too died.

In 2009, a second attempt was made to clone the Pyrenean Ibex; one
clone was born alive, but died seven minutes later, due to physical
defects in the lungs.

The concept of cloning extinct species was thought to be
popularized by the successful novel and movie Jurassic Park.